Abstract
Sub-bottom profiling (SBP) surveys and bathymetric mapping conducted off the shore of Miyako-jima, which belongs to the southern Ryukyus in the Ryukyu Island Arc, have revealed the presence of mound-shaped structures 3–8 m high and 50–120 m wide at depths of 70–100 m. The SBP surveys showed that the mounds possess strong distinct, convex upward reflector shapes at the top, which we interpret as submerged reefs and reefal sediments. Additionally, modern stratified sediment layers that cover these mound-shaped structures indicate that those reefs began forming and advancing shoreward in a back-stepping fashion as a result of sea-level rise. An analysis of the mound distribution shown by SBP and multibeam echo sounding (MBES) surveys suggest that they might have been formed during the lowstand stage of sea-level change, which includes the Last Glacial Period, because the distribution of these mounds is limited to water depths of 70 to 100 m, deeper than where present-day reefs grow. The SBP images hint that such high-resolution seismic profiles, accompanied by detailed bathymetric mapping off the reefal area, have the potential to provide effective indicators of not only coral reef paleoenvironment development, but also the tectonic setting of this offshore area.
Highlights
The Ryukyu Island Arc extends 1200 km from Kyushu to Taiwan along the Ryukyu Trench where the Philippine Sea Plate is subducting beneath the Eurasian Plate
We report on reef-like structures in this barren zone from 70 to 100 m deep in the central and southern Ryukyus based on new geological data obtained via a mapping project conducted by the Geological Survey of Japan (GSJ) of the National Institute of Advanced Industrial Science and Technology (AIST) in waters around Miyako-jima during their 2016 cruise, the results of which have the potential to contribute to our understanding of coral reef formation and the response of coral reefs to postglacial sea-level rises in a coral reef province of a relatively high-latitude area
3 Methods A GSJ research cruise (GH16) was conducted around Miyako-jima from July 28 to August 7, 2016, aboard the research vessel (R/V) Hakurei, operated by the Japan Oil, Gas and Metals National Corporation (JOGMEC). During this geological mapping project, we focused on water depths of more than 50 m and acquired more than 1878 nautical miles of highresolution multichannel seismic (MCS) profile data
Summary
The Ryukyu Island Arc extends 1200 km from Kyushu to Taiwan along the Ryukyu Trench where the Philippine Sea Plate is subducting beneath the Eurasian Plate. Miyako-jima belongs to the southern Ryukyus in the Ryukyu Island Arc and is located approximately 100 km east-northeast of Ishigaki-jima (Fig. 1). This island, about 30 km long, is triangle-shaped and surrounded by the small islands of O-gami-jima, Ikema-jima, Irabu-jima, Shimoji-shima, and Kurima-jima. This study focuses on the area off the northeastern coast of Miyako-jima. Off the central part of the northeastern coast lies Tsufutsuwa Reef (Tsufutsuwa Bise), a 2.5 km long patch reef (Coral Reefs of Japan 2004). Toward the northern end of this coast, there is a group of patch reefs around Ikemajima and O-gami-jima. The Yaebishi Reefs, developed north of Ikema-jima as shown, consist of approximately 100 table and platform reefs scattered in the offshore area The Yaebishi Reefs, developed north of Ikema-jima as shown in Fig. 2, consist of approximately 100 table and platform reefs scattered in the offshore area
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