[H-CG21] Earth surface processes related to deposition, erosion and sediment transport

[HCG21-P10] Identification of event deposits interbedded with varved sediment in Lake Ichinomegata

*Atsuki Nagata¹, Hiroto Kajita², Hirofumi Tazoe³, Junpei Abo², Hinano Kusunose², Takuto Ando⁴, Takahiro Suzuki⁴, Masahiro Minowa⁵, Iroha Ono⁶, Hitoshi Hasegawa⁶, Aki Sakuma⁷, Asuka Yamaguchi⁸, Yoshiaki Suzuki⁹, Koji Umeda² (1.Graduate school of Sustainable Community Studies, Hirosaki University, 2.Graduate school of Science and Technology, Hirosaki University, 3.Institute of Radiation Emergency Medicine, Hirosaki University, 4.Graduate School of International Resource Sciences, Akita University, 5.Institute of Low Temperature Science, Hokkaido University, 6.Faculty of Science and Technology, Kochi University, 7.Department of Earth and Planetary Science, School of Science, the University of Tokyo, 8.Atomosphere and Ocean Research Institute, The University of Tokyo, 9.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

Keywords:varve, turbidite, Lake Ichinomegata

Varves are annually laminated sediments formed by seasonal changes in geological and biological depositional processes. Varved sediment in lakes potentially provides high-resolution climatic and environmental archives in the catchment areas. Furthermore, in some cases, varved sediment sequences are interrupted by subaquatic mass movements, triggered by natural hazards such as earthquakes and hydrological events (runoff).
Lake Ichinomegata is a small (600m in diameter) and deep (approximately 45m in depth) maar in the Oga peninsula, Akita Prefecture. The detailed stratigraphy and chronology of the Lake Ichinomegata sediments have been thoroughly elucidated, enabling the determination that annually laminated sediments have been deposited continuously over the past 30ka. Consequently, the lacustrine sediments of Lake Ichinomegata offer the potential to elucidate a high-resolution Holocene seismic history that occurred in the coastal regions of the Japan Sea on Honshu Island.
We collected several short (approximately 40 cm in length) sediment cores from the central part of Lake Ichinomegata. Based on visual observation, macroscopic observation, soft X-ray image analysis, ¹³⁷Cs and excess ²¹⁰Pb age models, grain size analysis, XRF core scanning, µXRF measurements, and µXRD measurements, we identified four event layers (E1, E2, E3, E4) interbedded with the varved sediments.
Varves in Lake Ichinomegata generally consist of light and dark layers. The light layer is rich in clastic particles with high concentrations of Mn and Fe, while the dark layer is rich in organic matter and diatom shells with low concentrations of Mn and Fe. Therefore, we establish a verve counting age model based on visual observation and XRF patterns, which is almost consistent with the ²¹⁰Pb/¹³⁷Cs dating result. Based on the accurate chronology, E1 and E3 can be correlated to the 1983 Sea of Japan earthquake and the 1964 off Oga Peninsula earthquake, respectively. E2 and E4 can be correlated to the heavy rainfall disaster in 1979CE and 1955CE, respectively. The grain size distribution in the event layers differed according to the type of disaster (earthquake and heavy rainfall), which may reflect differences in sedimentary processes.
Our study suggests that the timing and type of previous natural hazards can be distinguished from the sediment core of Lake Ichinomegata over the last 30ka.