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

2019 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 / Fossil coral communities / Sea-level change

Outline of Annual Research Achievements

During FY2019 the present research project focused on the description and interpretation of Quaternary fossil coral assemblages in the Ryukyu Islands based on examination of both outcrops and cores.
Outcrops of late Middle Pleistocene coral limestones were examined at Horikawa quarry, southern Okinawa-jima, in October 2019. This study is part of a Nagoya Univ. student’s Master thesis and is conducted in collaboration with Prof. Kazuhiko Fujita (Univ. of the Ryukyus). The purpose of this study is to reconstruct the depositional history of the limestones of the late Middle Pleistocene Minatogawa Formation based on our understanding of the distribution of modern biota and sediments in the Ryukyu Islands. A detailed description of the coral species taxonomic and morphologic compositions of the limestones was carried out. Samples were collected for taxonomic and sedimentological analyses. Data from the four transects combined with those of previous surveys suggest a sea-level history more complex than previously thought.
In addition, the description of five cores taken from Kodakara-jima, South Ryukyus, began in July 2019. The cores were drilled by the COREF project and are now stored at the Kochi Core Center. The purpose of this study is to examine the responses of high-latitude coral communities to sea-level and environmental changes during the various stages of Holocene reef development.
In parallel to the aforementioned studies, analysis of the video images of mesophotic seafloor acquired along the margins of the Sekisei Lagoon, South Ryukyus, during past surveys is ongoing.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

The study of the COREF cores is slightly delayed due to additional time spent on the investigation of outcrops of Pleistocene coral limestones in southern Okinawa-jima.

Strategy for Future Research Activity

The following research activities related to the present project are expected to take place during FY2020:
(1) The study of the cores from Kodakara-jima will continue this year. This is now the main theme of a Master thesis at Nagoya University. The description of the cores will be carried out at the Kochi Core Center.
(2) An additional field survey in Southern Okinawa-jima targeting the limestones of the Minatogawa Formation is being planned. The survey will focus on stratigraphic intervals which have not yet been studied in detail.
(3) One more survey of the modern mesophotic seafloor around the Sekisei Lagoon using robotic submersibles (i.e., ROV, AUV) will be scheduled in order to complement the existing data base on the distribution of biota and sediments at mesophotic depths around the Sekisei Lagoon.
(4) A fossil mesophotic reef sequence sampled by IODP Exp. 325 will be examined at the Kochi Core Center to study the response of mesophotic coral communities to rapid deglacial sea-level rise.

Causes of Carryover

The budget for FY2020 will be used to conduct core description and sampling at the Kochi Core Center, as well as for the study of late Middle Pleistocene outcrops in southern Okinawa-jima. In addition, another ROV-AUV survey around the Sekisei Lagoon is planned.

Research Products

• [Presentation] Study of macrobenthic community structure at mesophotic depths in the South Ryukyus based on ROV and AUV seafloor imaging (2019)
  • Author(s): HUMBLET, M., MAKI, T., FURUSHIMA, Y., NOGUCHI, Y., HORIMOTO, H., YATAGAI, H., SAKAMAKI, T., ITO, S. and SUSUKI, S.
  • Organizer: Japanese Geoscience Annual Meeting