Study of reef biota and sediment distribution in the South Ryukyus and application to paleoenvironmental interpretation of Quaternary fossil reef deposits

2018 Fiscal Year Research-status Report

• Project/Area Number: 18K03824
• Research Institution: Nagoya University
• Principal Investigator: HUMBLET Marc 名古屋大学, 環境学研究科, 特任准教授 (40623616)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: Coral reefs / Mesophotic habitats / Benthic communities / Seafloor topography / AUV / ROV

Outline of Annual Research Achievements

An improved version of the lightweight autonomous underwater vehicle (AUV) HATTORI (Highly Agile Terrain Tracker for Ocean Research and Investigation) was built by Prof. Toshihiro Maki’s laboratory (Institute of Industrial Science, The University of Tokyo) in collaboration with FullDepth Co., Ltd. This new AUV (HATTORI 2), like its predecessor (HATTORI 1), is equipped with high-resolution video cameras and designed to take video images of the seafloor while following a pre-programmed path at a specified and constant elevation. HATTORI 2 can travel back and forth and therefore can map wider areas of seafloor compared with the previous model which could only travel in one direction. Moreover HATTORI 2 is equipped with a super short baseline acoustic system (SSBL) to determine its position relative to the ship during each dive. The new AUV HATTORI 2 was successfully tested in November 2018 at three sites around the Sekisei Lagoon, southern Ryukyu Islands. In addition, to further constrain the nature and distribution of marine organisms and sediments at mesophotic depths around the Sekisei Lagoon, two remotely operated vehicles (BlueRov 2, Blue Robotics Inc., and FullDepth DiveUnit 300, FullDepth Co., Ltd.) were deployed at six sites, including the two sites surveyed by the AUV at depths of 30-65 m along the outer margin of the Sekisei Lagoon and comprise 4 sites which, to the knowledge of the author, have never been explored previously. A total of ten sediment grab samples were also collected at the surveyed sites to verify the nature of bottom sediments.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

The AUV-ROV survey around the Sekisei Lagoon has taken place according to the initial plan. Data are now being processed. The next phase of the plan is to conduct a study of the fossil coral assemblages observed in cores taken at multiple locations in the Ryukyu Islands and stored at the Kochi Core Center. This core study was initially planned to take place in FY2018 but had to be postponed to FY2019 due to conflicting schedule with a geologic field survey related to an international research project in which the principal investigator became involved recently.

Strategy for Future Research Activity

The next phase of this research project will focus on the study of fossil coral assemblages in cores drilled at multiple locations in the Ryukyu Islands by the ICDP COREF project. There is a total of 11 cores targeted for this research, taken from three islands, in the south (Yonaguni-jima) and central (Okinawa-jima and Kodakara-jima) Ryukyu Islands. These cores are now stored at the Kochi Core Center. Fossil corals will be described at the lowest taxonomic level possible. Variations in taxonomic and morphologic compositions of fossil corals will be interpreted in terms of paleoenvironmental and paleo-water depth changes based on our current knowledge of the distribution of coral species on modern reefs in the same region, derived from the data acquired during this research project and from the scientific literature. In addition, samples of coralline algal crusts will be taken to complement the analysis of coral assemblages and their paleoenvironmental interpretation. Due to their wide latitudinal separations, analysis of coralgal assemblages in these cores is expected to shed light on the responses of reef-building communities to sea-level and environmental changes near the limit of their latitudinal range of distribution.
Depending on the timing of the core study mentioned above, the next ROV-AUV survey around the Sekisei Lagoon is expected be schedule toward the end of FY2019 or during FY2020.

Causes of Carryover

The budget for FY2019 will be used to conduct core description and sampling at the Kochi Core Center. In addition, another ROV-AUV survey around the Sekisei Lagoon is expected to take place during FY2019, or will be scheduled at latest during the first half of FY2020.

Research Products

• [Journal Article] Low-altitude and high-speed terrain tracking method for lightweight AUVs (2018)
  • Author(s): Maki, T., Noguchi, Y., Kuranaga, Y., Masuda, K., Sakamaki, T., Humblet, M., and Furushima, Y.
  • Journal Title: Journal of Robotics and Mechatronics Volume: 30 Pages: 971-979
  • DOI: 10.20965/jrm.2018.p0971
  • Peer Reviewed
• [Book] Ryukyu Islands, Japan (in: Mesophotic Coral Ecosystems, Coral Reefs of the World 12) (2019)
  • Author(s): Sinniger, F., Harii, S., Humblet, M., Nakamura, Y., Ohba, H., and Prasetia, R.
  • Total Pages: xx-xx
  • Publisher: Springer Nature
  • ISBN: 978-3319927343