Tide influenced high energy environments and rhodolith-associated carbonate deposition on the outer shelf and slope off the Miyako Islands, southern Ryukyu Island Arc, Japan

Abstract

Sediments distributed off the Miyako Islands, southern Ryukyus, Japan, include rhodoliths, larger foraminifera ( Cycloclypeus), and/or bryozoan dominated carbonates of low mud content on the outer insular shelf and planktonic foraminiferal mud free carbonate sand on the slopes. Sidescan sonograms show large scale 2-D subaqueous dunes in the deeper shelf sediments which suggest a NW-SE flow direction. Sea bottom photographs of the slope sediments to depths of 1000 m, show small scale 2-D or 3-D ripples. Those sedimentologic features suggest the existence of relatively high-speed currents on both the shelf and slope in the area. Current direction and velocity have been directly measured to clarify the origin and character of the current governing the area where the rhodoliths/large foraminifera are distributed. The results of measurements carried out at 14 stations at depth ranges of 14 to 799 m show that the direction and velocity of the currents on the shelf and slope change semi-diurnally. At most shelf stations, the currents at high tide are to the north-northwest, i.e., from the Pacific Ocean to the East China Sea. Conversely, the currents at low tide are to the south-southeast, from the East China Sea to the Pacific Ocean. On the slope, current movements are to the west at high tide and to the east at low tide. It is concluded, therefore, that the currents on the shelf and slopes are tidally related. Current speed is small in protected areas, and is high on the open shelf and shelf slopes. The Ryukyu Island Arc interrupts the tidal northwestern and southeastern flow. This interaction probably develops substantial turbulence and topography induced upwelling, and therefore forms a natural site for the formation of mud free and/or coarse sediments with rhodoliths and large formainifera.