C01_P01

U/Th dating and radiocarbon measurement potentials using marine mollusks around Japanese archipelago

Hirabayashi S¹, Aze T¹, Miyairi Y¹, Kan H², Yokoyama Y¹

¹Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan, ²Research Center for Coastal Seafloor, Kyushu University, Nishi-ku, Japan

Radiocarbon dating of mollusks are widely used in paleoclimatology and archeology, while Uranium-thorium (U/Th) dating is generally difficult to provide reliable age using mollusks. However, several recent studies suggested that the U/Th dating potential using marine bivalves in Mediterranean Sea, Caspian Seaand Korea. These studies have investigated using limited mollusks species and regions.

In this study, we investigated the distribution of radiocarbon and uranium in mollusks, including the calcified opercula of Turbo sazae Fukuda, Turbo marmoratus and Tridacnina sp. collected from Ryukyu region and Chiba, Japan, to evaluate the possibility of U/Th dating potentials using marine mollusks around Japanese archipelago. We measured high-resolution radiocarbon and uranium concentration using single stage AMS (Yokoyama et al., 2019) and laser-ablation ICP-MS in Atmosphere and Ocean Research Institute, The University of Tokyo, respectively. Our results showed that uranium in opercula of modern Turbo sazae Fukuda and Tridacnina sp. were unevenly distributed and those concentration were 1000 times-less than that in coral skeletons, while radiocarbon in the shell samples were reflected the radiocarbon values in the ambient seawater as well as corals. The uranium in the calcified opercula of Holocene Turbo marmoratus was also unevenly distributed but concentrated area in the opercula was different from that of the modern samples, which suggested exchange uranium after they were deposited. Our results suggested that the uptake processes of radiocarbon and uranium isotopes into mollusks shell were different, and it is important to understand the criteria of choosing the mollusks species for U/Th dating around Japan.