14C Ages of Samples Collected from Conglomeratic Beachrocks Developed at the Northern Coast of Geruma Island, West of Okinawa Island

Conglomeratic beachrocks develop on the northern coast of Geruma Island, located to the west of Okinawa Island. They are divided into seven rows, three of which lie below the mean low tide level. In order to determine formation ages of conglomeratic beachrocks four fine sandy and muddy calcareous deposits (cement) consolidating cobbles and boulders were collected from four landward rows. They were radiocarbon dated with Accelerator Mass Spectrometry (AMS) and yielded formation ages of 1192 & PLUSMN; 58 yr BP, 2842 & PLUSMN; 61 yr BP, 4661 & PLUSMN; 63 yr BP, and 5240 & PLUSMN; 65 yr BP from landward to seaward rows, respectively. Elemental analyses to determine Ca concentration of cements were conducted with Inductively Coupled Plasma Mass Spectrometry (ICP-MS) resulting in values of 24.6% for the eastern beachrock, and 31.2% for the western beachrock, which are equivalent to 61.6% and 78.2% of CaCO3, respectively. These high CaCO3 concentrations made it possible to form the conglomeratic beachrocks that developed on the northern coast of Geruma Island.