A newly discovered submerged reef on the Miyako-Sone platform, Ryukyu Island Arc, Northwestern Pacific

Abstract

Bathymetric echosounder mapping and seafloor observations using a remotely operated vehicle were conducted on the Miyako-Sone platform, located northeast of Miyako-jima, Ryukyu Islands, northwestern Pacific Ocean. This study aims to unravel the origin and development of the Miyako-Sone platform, a key field to reconstruct the biogeographic evolution and the geologic history of the Ryukyu Islands. The resulting high-resolution bathymetric map reveals details of the seafloor geomorphology, including the spatial distribution of submerged terraces on the platform. We report herein the discovery of a submerged reef (the Miyako-Sone reef) on the platform whose top is at a water depth of 56m, extending in a north–south direction for up to 1km and with a minimum width of 500m. The reef consists of a central depression and a concentric marginal ridge; topographic features analogous to “spurs and grooves” are recognized along the outer margin of the marginal ridge. The Miyako-Sone reef was drowned during a period of global postglacial sea-level rise between ~11–12ka which may coincide with MWP-1B at ~11.5ka. The increase in sea surface temperature in the Ryukyu Islands at ~11ka shows that the Miyako-Sone reef formed during the final stage of the cooler postglacial period. Part of the Miyako-Sone platform was likely a land bridge for the migration of terrestrial organisms from Okinawa-jima to Miyako-jima during low sea-level stands after the deposition of the Ryukyu Group.