5:15 PM - 6:45 PM
[HQR05-P05] Relationships between paleoenvironmental changes and sea-level shift during the past 6000 years in Lake Kasumigaura
Keywords:Lake Kasumigaura, Grain size, Paleoenvironment, Tidal current, Holocene
Lake Kasumigaura, located in the southern part of Ibaraki Prefecture, is a freshwater lake consisting of Nishiura, Kitaura and Sotonasakaura. The Lake had been formed by the development of sandbars in the ocean. The area around Lake Kasumigaura was ko-kinu Bay due to the sea-
level rise of the Jomon transgression. Then, it changed to brackish water lake gradually since the bay mouth became close (Saito et al., 1990) by sediments of the Kinu River (Tanabe et al., 2022). However, the paleoenvironmental reconstructions of Lake Kasumigaura so far have mainly been conducted on several thousand years. Addition, there was no study for the paleoenvironments on a scale of several hundred years, especially in the northwestern part of the lake. Our purpose of this study is to reconstruct the paleoenvironment of Lake Kasumigaura (Nishiura) for the past 6,000 years on a high resolution. Details of two cores collected in the southeast area are reported by Yamada et al. (H-QR05). In this poster, the paleoenvironmental changes in the whole area of Lake Kasumigaura (Nishiura) are discussed together with the results of the core in southeastern part.
We collected the KS23-02 core by using a percussion piston corer in the northwestern part of Lake Kasumigaura (Nishiura) in June 2023. The core was examined for the sedimentary structure by CT scan images, and for the grain size analysis of 421 samples of 1 cm thick slices. The core length is 422 cm and is composed mainly of silt intercalated some tephra layers. The core depth of 350–225 cm consisted of mud with thin fine-grained sand layers frequently. The sediments in the core depth of 225–120 cm consisted of bioturbated mud. The five tephra layers including the Asama A, were recognized on the basis of magnetic susceptibility and microscopic observation. Based on the tephra layers and 14C dating of bivalves from the core depth of 375 cm, the age model of the core was calculated. The four paleoenvironments were identified; a mud bottom in a closed inner bay during 3500–2500 cal yr BP, the bay influenced by tidal current between 2500–1200 cal yr BP, the bay with weak tidal effect in 1200–600 cal yr BP, and the closed brackish or freshwater lake after 600 cal yr BP.
Fine-grained sand was recognized after 2500 cal yr BP, whereas only mud was deposited in the northwestern part of Lake Kasumigaura between 3500 and 2500 cal yr BP. The dominance of mud in this interval was identified in the two cores from southeastern part. In the Kanto area, it is known that the about 4 m sea-level fall called the "Yayoi regression" occurred between 4000 and 3000 cal yr BP (Tanabe et al., 2016, etc.). The common mud dominance in the Lake Kasumigaura suggests that the entire area in Lake Kasumigaura (Nishiura) became a closed environment because of the enclosed the bay mouth due to the sea-level fall.
level rise of the Jomon transgression. Then, it changed to brackish water lake gradually since the bay mouth became close (Saito et al., 1990) by sediments of the Kinu River (Tanabe et al., 2022). However, the paleoenvironmental reconstructions of Lake Kasumigaura so far have mainly been conducted on several thousand years. Addition, there was no study for the paleoenvironments on a scale of several hundred years, especially in the northwestern part of the lake. Our purpose of this study is to reconstruct the paleoenvironment of Lake Kasumigaura (Nishiura) for the past 6,000 years on a high resolution. Details of two cores collected in the southeast area are reported by Yamada et al. (H-QR05). In this poster, the paleoenvironmental changes in the whole area of Lake Kasumigaura (Nishiura) are discussed together with the results of the core in southeastern part.
We collected the KS23-02 core by using a percussion piston corer in the northwestern part of Lake Kasumigaura (Nishiura) in June 2023. The core was examined for the sedimentary structure by CT scan images, and for the grain size analysis of 421 samples of 1 cm thick slices. The core length is 422 cm and is composed mainly of silt intercalated some tephra layers. The core depth of 350–225 cm consisted of mud with thin fine-grained sand layers frequently. The sediments in the core depth of 225–120 cm consisted of bioturbated mud. The five tephra layers including the Asama A, were recognized on the basis of magnetic susceptibility and microscopic observation. Based on the tephra layers and 14C dating of bivalves from the core depth of 375 cm, the age model of the core was calculated. The four paleoenvironments were identified; a mud bottom in a closed inner bay during 3500–2500 cal yr BP, the bay influenced by tidal current between 2500–1200 cal yr BP, the bay with weak tidal effect in 1200–600 cal yr BP, and the closed brackish or freshwater lake after 600 cal yr BP.
Fine-grained sand was recognized after 2500 cal yr BP, whereas only mud was deposited in the northwestern part of Lake Kasumigaura between 3500 and 2500 cal yr BP. The dominance of mud in this interval was identified in the two cores from southeastern part. In the Kanto area, it is known that the about 4 m sea-level fall called the "Yayoi regression" occurred between 4000 and 3000 cal yr BP (Tanabe et al., 2016, etc.). The common mud dominance in the Lake Kasumigaura suggests that the entire area in Lake Kasumigaura (Nishiura) became a closed environment because of the enclosed the bay mouth due to the sea-level fall.