[S-SS12] Active faults and paleoseismology

[SSS12-07] A critical evaluation for High-Precision Dating Method on Hydroides Ezoensis to understand Crustal Deformation History in Tohoku Region, Japan

★Invited Papers

*Kai Leggett¹, Yusuke Yokoyama¹, Yoshiki Shirahama², Yosuke Miyairi¹, Naoto Fukuyo¹, Kosuke Ota¹, Kohei Abe³, Shuji Terusawa³, Tomoo Echigo⁴ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Advanced Industrial Science and Technology, Geological Survey of Japan (currently at Ministry of Education, Culture, Sports, Science and Technology), 3.Oyo Corporation Ltd, 4.Kankyo Chishitsu Co.Ltd)

Keywords:Radiocarbon, Crustal Movement History, Hydroides , Accelerator Mass Spectrometer Analysis, Tohoku coastal region

Uplift and subsidence due to crustal movement occur in the coastal areas of the Tohoku region, especially along the Pacific Ocean region where uplift and subsidence occur in cycles of several thousand years (Ikeda et al., 2012). However, there are limited methods available to reconstruct the crustal movement history with high resolution in the Tohoku coastal area. The paleo shoreline indicator, Pomatoleios Kraussii, has not been used in the Tohoku region because its habitat is limited to the south of the Kanto region, and it is unclear whether it is suitable for radiocarbon (¹⁴C) dating due to the complex marine environment. In the Tohoku region, where the Oyashio and Kuroshio currents mix in winter on the Pacific side, these currents hold different radiocarbon concentrations (Δ¹⁴C). This mixture of different Δ¹⁴C creates large uncertainty in carbon age. In this study, we developed a ¹⁴C dating method for Hydroides Ezoensis, an intertidal annelid that inhabits in Tohoku coastal region, to evaluate its use in reconstructing crustal movement history in the Tohoku region. Although previous studies have demonstrated that the Hydroides Ezoensis can accurately determine the elevation of paleo shorelines (Miura & Kajiwara, 1983), its suitability as a dating method remains unclear.
Here, we evaluated the Δ¹⁴C of Hydroides Ezoensis on the Pacific and Japan Sea sides of the Tohoku region with values reported in previous studies. First, samples were identified by stereomicroscope. Secondly, we evaluated the suitability of calcareous tubes for ¹⁴C dating by comparing them with Δ¹⁴C of shellfish and seawater reported in previous studies. SEM and XRD analyses were conducted to identify and remove samples with mineral alteration affecting the ¹⁴C age. In addition, we conducted radiocarbon dating of fossil Hydroides Ezoensis calcareous tubes collected from uplifted coastal landforms in the Tohoku region and compared them with historical earthquake records based on their high precision elevation and age. Our results show that (1) the samples obtained in this study, which still have external characteristics, are the calcareous tubes of Hydroides Ezoensis. This suggests that stereomicroscope analysis of calcareous tubes can be reliably used in identifying the species. (2) The Δ¹⁴C recorded by Hydroides Ezoensis only reflects the Kuroshio Current Δ¹⁴C and is useful as a dating sample. (3) The fossilized calcareous tube of the Hydroides Ezoensis has not been altered enough to influence the ¹⁴C dating. (4) The ¹⁴C age and elevation of the Hydroides Ezoensis are consistent with recorded elevation changes from historical seismic records.
Overall, this study suggests that Hydroides Ezoensis can be used to reconstruct the crustal movement history of the Tohoku region with as much precision as Pomatoleios Kraussii.