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[MIS23-P04] Weathering environment after the last glacial period around the Lake Suwa, Nagano Prefecture, central Japan
Keywords:chemical composition analysis, chemical weathering, paleoenvironmental restoration
Reconstruction of the paleoenvironment around Lake Suwa had been carried out by Anma et al. (1990) and Oshima et al. (1997). In these studies, vegetation-transitions and sedimentary environments have been estimated using diatom and pollen fossil communities. On the other hand, it is known that the chemical composition of sediments reflects the weathering process and source rock types. Therefore, in this study, we examined chemical analysis of sediments of boring core excavated on the southern shore of Lake Suwa.
The core used in this study reached a depth of 30 m, and the upper 5 m was an artificial sediment. The core lithology indicates that the sedimentary environments changed from the bottom of the lake near the channel, the bottom of lake, finally to the river channel in ascending order. The age of oldest sediments was estimated to be about 22000 years ago by the 14C dating method. The sediments were divided into 5 units, and 110 samples were collected from the core for XRF and XRD analysis. In addition, the minerals in the river sands, which were collected from the bedload sediments around Lake Suwa, were examined for model analysis using a microscope.
The sediments of the inflowing rivers were mostly occupied by weathered volcanic rock fragments. In the core sediments, result of XRD indicated that most sediments consisted of a large amount of glassy and/or amorphous material, except for unit3. From these results, except for unit3, the sediments of Lake Suwa are considered to be derived from the surface soil of the catchment area, and it is presumed that they were mainly derived from the Kamigawa river, which currently mainly supplies volcanic rock fragments. The chemical traditional trend in the A-CN-K diagram and A-CNK-FM diagram proposed by Nesbitt and Young (1982) suggested that unit4 was derived from more mafic rocks, whereas other units were suppled from andesitic rocks. Furthermore, unit3 has a high CIA value (Nesbitt and Young, 1982) with kaolinite, suggesting that the strong weathered products originated from felsic source rocks. In addition, several changes of weathering condition and source rocks imply temporal climate change in 11700-10500yrBP.
References
Anma, K., Nagaoka, M., Niwa, S., Sekimoto, K,, Yoshikawa, M., Fujine, H., 1990, A Geological Survey for the Study on Neotectnic Movement and Geoenvironment of Lake Suwa, Nagano Prefecture, Mem. Geol Soc. Japan, 36, 179-194.
Nesbitte, H. W. and Young, G. M., 1982, Early Proterozoic climates and plate motions inferred from major element chemistry of lutites, Nature Vol.299, 715-717.
Oshima, H., Tokunaga, S., Shimokawa, K., Mizuno, K., Yamazaki, H., 1997, Fossil Pollen Assemblages of Core Samples from Lake Suwa, Nagano Prefecture, and Their Correlation to Other Pollen Assemblages in Central Japan, The Quaternary Research 36(3), 165-182.
The core used in this study reached a depth of 30 m, and the upper 5 m was an artificial sediment. The core lithology indicates that the sedimentary environments changed from the bottom of the lake near the channel, the bottom of lake, finally to the river channel in ascending order. The age of oldest sediments was estimated to be about 22000 years ago by the 14C dating method. The sediments were divided into 5 units, and 110 samples were collected from the core for XRF and XRD analysis. In addition, the minerals in the river sands, which were collected from the bedload sediments around Lake Suwa, were examined for model analysis using a microscope.
The sediments of the inflowing rivers were mostly occupied by weathered volcanic rock fragments. In the core sediments, result of XRD indicated that most sediments consisted of a large amount of glassy and/or amorphous material, except for unit3. From these results, except for unit3, the sediments of Lake Suwa are considered to be derived from the surface soil of the catchment area, and it is presumed that they were mainly derived from the Kamigawa river, which currently mainly supplies volcanic rock fragments. The chemical traditional trend in the A-CN-K diagram and A-CNK-FM diagram proposed by Nesbitt and Young (1982) suggested that unit4 was derived from more mafic rocks, whereas other units were suppled from andesitic rocks. Furthermore, unit3 has a high CIA value (Nesbitt and Young, 1982) with kaolinite, suggesting that the strong weathered products originated from felsic source rocks. In addition, several changes of weathering condition and source rocks imply temporal climate change in 11700-10500yrBP.
References
Anma, K., Nagaoka, M., Niwa, S., Sekimoto, K,, Yoshikawa, M., Fujine, H., 1990, A Geological Survey for the Study on Neotectnic Movement and Geoenvironment of Lake Suwa, Nagano Prefecture, Mem. Geol Soc. Japan, 36, 179-194.
Nesbitte, H. W. and Young, G. M., 1982, Early Proterozoic climates and plate motions inferred from major element chemistry of lutites, Nature Vol.299, 715-717.
Oshima, H., Tokunaga, S., Shimokawa, K., Mizuno, K., Yamazaki, H., 1997, Fossil Pollen Assemblages of Core Samples from Lake Suwa, Nagano Prefecture, and Their Correlation to Other Pollen Assemblages in Central Japan, The Quaternary Research 36(3), 165-182.