Abstract


HIGH-ENERGY WINDOW IN MODERN REEFS: AN EXAMPLE FROM MULTIBEAM BATHYMETRIC MAPPING OF REEF GEOMORPHOLOGY AROUND THE RYUKYU ISLANDS

Accurate descriptions of coral reef geomorphology improve our understanding of reef environments. Since 2010 we have been conducting high-resolution multibeam bathymetric surveys around the Ryukyu Islands, southern Japan, and mapping the coral reef geomorphology with a horizontal grid size of 1–2 m. Here, we introduce our mapping project, which covers of a total of 39 surveyed areas across five islands, and present an example illustrating the barrier reef and island shelf geomorphology of eastern Kume Island (1.8 × 6.5 km bathymetric area, with a depth range of 0.4–161.3 m) combined with SCUBA and VTR observations. The bathymetric area comprises two contrasting reef edge formations: a double reef in the western area, and a deep breakwater reef in the eastern area. The latter shows the high-energy window condition which is accompanied by spur and groove formation in the lagoon, formation of circular spits or bowl-shaped depressions behind the lagoon patch reefs, and traces of movement of coral boulders in the lagoon following typhoon storm surges. These observations enable us to understand and reconstruct the paleo-geomorphology and sedimentology in reefs associated with the Holocene high-energy window, and can also contribute to predictions of coastal environmental change associated with future sea level rise. On the island shelf, we observed five reef terraces at depths between 80 and 95 m, and isolated patch reefs at 135 m depth. The discovery of these features may contribute to reconstructions of past sea level and reef growth in the northwestern Pacific.

Authors

KAN, H., Kyushu University, Japan, kan@scs.kyushu-u.ac.jp

FUJITA, K., University of the Ryukyus, Japan, fujitaka@sci.u-ryukyu.ac.jp

HASEGAWA, H., Kokushikan University, Japan, hasegawa@kokushikan.ac.jp

NAGAO, M., Institute of Geology and Geoinformation, AIST, Japan, nagao-masayuki@aist.go.jp

NAKASHIMA, Y., Ariate National College of Technology, Japan, nakasima@ariake-nct.ac.jp

HORI, N., Nara University, Japan, gaohorin@gmail.com

Details

Oral presentation

Session #:34
Date: 06/23/2016
Time: 15:15
Location: 311

Presentation is given by student: No