Reconstruction of late Quaternary sea-level and reef growth history in the Ryukyu Islands

2022 Fiscal Year Annual Research Report

• Project/Area Number: 21H01169
• Research Institution: Nagoya University
• Principal Investigator: HUMBLET Marc 名古屋大学, 環境学研究科, 准教授 (40623616)
• Co-Investigator(Kenkyū-buntansha): 藤田 和彦 琉球大学, 理学部, 教授 (00343377) ; 井龍 康文 東北大学, 理学研究科, 教授 (00250671) ; 横山 祐典 東京大学, 大気海洋研究所, 教授 (10359648)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: Coral reef / Sea level change / Climate change / Quaternary / Okinawa / Minatogawa Formation / Limestone

Outline of Annual Research Achievements

During FY 2022 a second series of boreholes were drilled into the Minatogawa Formation in southern Okinawa Island as part of Grant-in-Aid for Scientific Research (B) project number 21H01169. Seven new boreholes were drilled in August and September 2022. The new core sections will complement those extracted from the five boreholes already drilled in December 2021 and January 2022 along a first transect perpendicular to the shoreline. The new drilling campaign recovered core sections from five 8-23 m long boreholes drilled along a second transect situated approximately one kilometer to the southwest of the first one, one 17 m long borehole drilled between the first and second transect, and one 10 m long borehole drilled along the first transect. The total cored length is 98 m. The new cores were shipped to the Kochi Core Center in January 2023 where they will be kept in a refrigerated storage room together with the 90 m of core sections of the first transect. In September and October 2022, the cores from the first transect were split in half lengthwise and their lithology, sedimentological features, and fossil content (particularly corals, coralline algae, and foraminifers) were described. Sampling of these cores was carried out in February 2022 for sedimentological and geochemical analyses, radiometric dating of corals, taxonomic identification of corals, coralline algae, and foraminifers. Meanwhile, the corals exposed in core sections were identified at the lowest taxonomic level possible and their context, i.e., whether in situ or not, was also determined.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

The drilling campaign in southern Okinawa was completed and a total of 188 m of core sections was recovered from twelve boreholes drilled into the Minatogawa Formation. The splitting, description, and sampling of the first batch of core sections were completed during FY 2022.

Strategy for Future Research Activity

Research activities during FY 2023 are focused on the description and sampling of the core sections retrieved from the seven new boreholes drilled into the Minatogawa Formation in August and September 2022. The cores from the seven new boreholes will be split lengthwise at the Kochi Core Center. The description and sampling of the new core sections will be carried out at the Kochi Core Center with the assistance of Professor Yasufumi Iryu (Tohoku University) and Professor Kazuhiko Fujita (University of the Ryukyus). The samples will be used for sedimentological and geochemical analyses of the core material, screening of well-preserved corals for radiometric dating, as well as the taxonomic identification of corals, coralline algae, and foraminifers.