Finding of a huge coral reef sliding down to the bottom of the Palau Trench

We found a huge limestone block from the Palau Trench bottom, southern Philippine Sea by submersible Shinkai 6500. The limestone consists of the shallow marine coral reef similar with that of the present coral reefs of the Palau Islands. The site of a huge limestone body is located at the southern part of the Palau Trench at the water depth of 6400 m. The size of coral reef is confirmed to be of 2km x 2 km x 1 km by submersible observation but bathymetric survey confirm the distribution of the coral reef to be of 20 km x 10 km x 3 km. The limestone of the coral reef shows striations by the fall down blocks. The surface of the limestone is dissolved nature because of the depth being deeper than that of the Carbonate Compensation Depth (CCD), ca. 4200 m in the western Pacific. The limestone body is intercalated by a black sediment and is covered by both calcareous planktonic and benthic foraminifers which indicate the very shallow marine environment. Age of the limestone is middle Miocene by the Sr isotope age determination as well as fossils in the limestone itself. The bathymetric survey revealed a huge horseshoe morphology now forming a submarine canyon structure nearby the limestone site. Gravity and magnetic survey show the notable anomaly for several seamounts on the Caroline Plate. We had a scenario that the coral reef was once exposed on land along the Palau arc then collapsed and sledded down to the trench bottom by the tectonic erosion of the forearc of the Palau Trench due to the subduction of seamounts on the Caroline Plate at sometime during Pleistocene. In the Palauan people have legends of their history making storyboards which tell us a story that the Palau Island was sinking.