5:15 PM - 6:45 PM
[HQR05-P04] Grain size changes in southeastern part of the Lake Kasumigaura during the last 6,000 years
Keywords:Lake Kasumigaura, Holocene, paleoenvironment, sea level change
Lake Kasumigaura, located in southeastern part of Ibaraki Prefecture, is a sea relic lake. Its average water depth is 3.4 m in the present. Lake Kasumigaura, in a broad sense, is composed of Nishiura, Kitaura and Sotonasakaura. Lake Kasumigaura (Nishiura) changed from closed bay to brackish lake until 6000 years ago, then, changed to the freshwater lake. In the previous study, paleoenvironments for the time scale of several hundred thousand years were recognized during the past 10,000 years (Saito et al.,1990). However, those in the short time scale were ambiguous for the past 6,000 years. Thus, our purpose is to reconstruct past environments for high resolution time scale by using two cores taken from southeastern part of the Lake Kasumigaura (Nishiura).
The three cores were taken from southeastern part (KS22-04 and KS23-01) of Lake Kasumigaura (Nishiura) and northwestern part (KS23-02) were collected by the percussion piston corer in 2022 and 2023. In this poster, the results from the two cores taken in the southwestern part are described. Comparisons of the results between the cores in southwestern part and that in northeastern part are discussed in the poster of Watanabe et al. (H-QR05).
Lithological observations, CT scan images and grain size analysis were performed to the sediments of the cores. KS23-01 core, taken from southeast of the KS22-04 core site, is 424 cm long. For KS23-01 core, very fine-grained sand with low angle laminations and bioturbation was found in the core depth deeper than 125 cm. Mud in which low angle lamination and mud having laminations inclined two sides were recognized in core depths of 60–125 cm and shallower than 60 cm, respectively. Corbicula bed was found in the core depth around 16 cm. The KS22-04 core was 401 cm long, and their sediments were composed of mainly dark and light gray colored mud containing shell fragments. Mud showing low angle laminations inclined two sides in the core depth deeper than 100 cm and parallel laminated mud containing shell fragments in the core depth shallower than 100 cm were recognized. Comparisons of the two cores indicated that sand was dominated in southeastern part of the study area, and mud was predominant in the northwestern part. Further, mean grain size and sand content decreased upward in both cores, suggesting that the sand had supplied from southeast of the lake through the bay mouth by the tidal current during 6000–1000 years ago. Then, the bay changed to the brackish lake of the mud bottom due to decrease of sand supply from the tidal current. The rapid drops in mean grain size and sand contents were common in the two cores in 4000–3000 years ago. The sea-level drop of 4 m found in and around the study area (Tanabe et al., 2016) in this interval might cause the closing of the bay mouth and predominant of mud in the southeastern part of the lake bottom.
The three cores were taken from southeastern part (KS22-04 and KS23-01) of Lake Kasumigaura (Nishiura) and northwestern part (KS23-02) were collected by the percussion piston corer in 2022 and 2023. In this poster, the results from the two cores taken in the southwestern part are described. Comparisons of the results between the cores in southwestern part and that in northeastern part are discussed in the poster of Watanabe et al. (H-QR05).
Lithological observations, CT scan images and grain size analysis were performed to the sediments of the cores. KS23-01 core, taken from southeast of the KS22-04 core site, is 424 cm long. For KS23-01 core, very fine-grained sand with low angle laminations and bioturbation was found in the core depth deeper than 125 cm. Mud in which low angle lamination and mud having laminations inclined two sides were recognized in core depths of 60–125 cm and shallower than 60 cm, respectively. Corbicula bed was found in the core depth around 16 cm. The KS22-04 core was 401 cm long, and their sediments were composed of mainly dark and light gray colored mud containing shell fragments. Mud showing low angle laminations inclined two sides in the core depth deeper than 100 cm and parallel laminated mud containing shell fragments in the core depth shallower than 100 cm were recognized. Comparisons of the two cores indicated that sand was dominated in southeastern part of the study area, and mud was predominant in the northwestern part. Further, mean grain size and sand content decreased upward in both cores, suggesting that the sand had supplied from southeast of the lake through the bay mouth by the tidal current during 6000–1000 years ago. Then, the bay changed to the brackish lake of the mud bottom due to decrease of sand supply from the tidal current. The rapid drops in mean grain size and sand contents were common in the two cores in 4000–3000 years ago. The sea-level drop of 4 m found in and around the study area (Tanabe et al., 2016) in this interval might cause the closing of the bay mouth and predominant of mud in the southeastern part of the lake bottom.