[S-GL21] Geochronology and Isotope Geology

[SGL21-P02] Holocene sedimentary environmental changes in the Haccho-daira Swamp in the Kyoto-Kitayama area

Ku Nishikawa¹, *Mitsuru Okuno¹, Mao Oshita¹, Toshiyuki Fujiki², Akira Hayashida³, Toshimichi Nakanishi⁴, Masayuki Torii⁵, Wan Hong⁶ (1.Osaka Metropolitan University, 2.Okayama University of Science, 3.Doshisha University, 4.Museum of Natural and Environmental History, Shizuoka, 5.Kumamoto University, 6.Korea Institute of Geoscience and Mineral Resources)

Keywords:Haccho-daira Swamp, Holocene, Kikai-Akahoya (K-Ah) tephra, Ulleung-Oki (U-Oki) tephra, Aira-Tn (AT) tephra

Haccho-daira Swamp is located at an altitude of 800 to 820 meters of the water source of the Egaya River in the Ado River water system flowing to Lake Biwa. The U-OKI and AT tephras have been detected from this wetland, and the environmental changes for the past 30 kyr has been revealed (Takahara and Takeoka, 1986). However, the K-Ah tephra (7.3 cal ka BP) has been not found, and it is necessary to consider the succession of sedimentary records. In this study, core samples were collected using a peat sampler from a total of seven locations, five locations from the wetland (about 200 m range) and two locations in branch valley. The lower horizon could not be collected because it hits the gravel layer near 1 m from the surface. First, the facies of core sample was observed, and then X-ray imaging, digital color measurement, initial magnetization measurement, elemental analysis (C, N, S), mud-content ratio measurement, and ¹⁴C dating were performed. A volcanic glass correlated with the AT was detected from around 85 cm in depth. The ¹⁴C ages of about 11 ka BP are obtained from the plant fragments in the clay layer at the bottom. The U-Oki and K-Ah could not be found in this study.
The Hatcho-daira Swamp is known to have three cycles of gravel layers, clay layers, and peat layers. This cycle corresponds to blockage by collapse of surrounding mountain slopes and transition from ponds to wetlands. In the last cycle in the Holocene, sedimentation was interrupted around K-Ah horizon, and there is a high possibility that no record remains (hiatus).