Evaluation of uranium-thorium dating and radiocarbon measurement potentials using marine mollusks
Dr Shoko Hirabayashi1, Dr Takahiro Aze1, Dr Yosuke Miyairi1, Dr Hironobu Kan2, Dr Yusuke Yokoyama1
1Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan, 2Research Center for Coastal Seafloor, Kyushu University, Fukuoka, Japan
Mollusks incorporate radiocarbon and uranium into their skeleton. 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 (Rowe et al., 2015), Caspian Sea (Arslanov et al., 2002) and Korea (Cheong et al., 2006). These studies have investigated limited mollusks species and regions. Therefore, the uranium uptake process into mollusks shell and the criteria of choosing the mollusks species for U/Th dating have not been understood.
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 Kume Island, Ryukyu region and Chiba, Japan. 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. The uranium concentrated in the center of opercula of Turbo sazae Fukuda and in the hinge and outer layer of Tridacnina sp. Those concentration were 1000 times-less than that in coral skeletons, which are widely used for U/Th dating. The uranium in the calcified opercula of Holocene (1 ka BP) 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. We will also discuss the possibility of U/Th dating and reconstruction of seawater mass mixing based on radiocarbon measurement using mollusks.
Dr. Shoko Hirabayashi is a lecturer in Atmosphere and Ocean Research Institute, The University of Tokyo.
Her research interest includes reconstructing oceanography in the Holocene and Anthropocene using Radiocarbon dating and Uranium-Thorium dating of calcium carbonates.