[H-GM03] Geomorphology

[HGM03-P06] Sedimentary processes on the sublacustrine plain in Lake Kitaura: implications from component separation analysis of grain-size distributions

*Naofumi Yamaguchi¹ (1.Global and Local Environment Co-creation Institute, Ibaraki University)

Keywords:grain-size distribution, logratio analysis, grain-size component separation, lake sediment

Japanese coastal lakes are known to have common bottom topography, such as a littoral shelf along the shore and a sublacustrine plain in the offshore area. Lake Kitaura, which is a coastal lake located in central Japan, also has a similar bottom topography. The sediments on the littoral shelf are well-sorted sand, suggesting the significant influence of waves. In contrast, muddy sediments of unknown supply processes are distributed on the sublacustrine plain. The present study investigated the origins and processes of sediment supply on the sublacustrine plain through detailed analysis of grain-size distributions. Bottom surface sediments were collected and analysed for grain size at a total of 32 sites on the sublacustrine plain around the lake centre in Lake Kitaura. The obtained grain-size distributions were separated into lognormal distribution components using the EM algorithm. The number of components was determined according to Yamaguchi (2023), and the components obtained were classified into four common component groups. Estimation of unchanging component and logratio analysis were applied to the mixing proportions of each component group. The results showed that regional differences within the lake were small for the logratio of the fine-grained component groups, suggesting biogenic particles produced in the lake, such as diatoms, and uniform diffusion due to repeated roll-up. In contrast, the logratio of the coarsest-grained component group showed large regional differences within the lake. The coarsest-grained component group was more abundant closer to the shore, suggesting that they were supplied from the coastal area by waves. In the mouth of the Nakasato River, which flows into the eastern part of the study area, the coarsest-grained component group was also larger than those at similar depths, suggesting sediment supply from the river.