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Keywords:

  • coral reefs;
  • high-resolution seismic survey;
  • Kuroshio;
  • Last Glacial Maximum;
  • Ryukyu Islands

Abstract

High-resolution single-channel seismic reflection surveys were carried out in the northern Ryukyu Island Arc during an NT05-14 cruise of the R/V Natsushima. The survey area is located northeast of Amami Oshima and southwest of Kikai Shima immediately south of the current northern limit of coral reef formation in the northwestern Pacific Ocean. The main purpose of the surveys was to ascertain whether coral reefs formed during glacial periods and, if so, to determine their three-dimensional distribution. We collected 784 km of high-resolution single-channel seismic data during the cruise. The acoustic basement is clearly observed under the Amami Spur, off Amami Oshima. The seismic profiles show two anticlines, each with an axis trending northeast to southwest. Thin Quaternary stratified sediment overlies the acoustic basement in the northwestern and southeastern slope areas, as well as between the anticlinal axes. The stratified sediment can be divided into several sedimentary units. The mound-shaped reflections are found within the stratified sedimentary units in the central to eastern parts of the spur. Because the mound-shaped reflections are characterized by strong reflections and chaotic internal structures in profiles, they are considered biogenic ‘reefs’ or banks consisting of coarse-grained bioclasts. They overlie the acoustic basement or stratified sediment and reach 15 m in thickness and 400 m in width. In contrast, irregularly shaped topographic highs were detected at the shelf edge southwest off Kikai Shima, which are likely to be remnants of coral reefs formed during the last glacial period. Our seismic data indicate probable coral reef formation at low stands during glacial stages, such as the Last Glacial Maximum, even in the northern Central Ryukyus. The occurrence of drowned reefs may indicate that their growth rate was not sufficient to keep up with a rapid rise in sealevel after a glacial period.