11:00 〜 13:00
[HCG25-P07] 諏訪湖における堆積物の風化と供給源変遷：最終氷期以降の堆積物の化学組成からの考察
キーワード：化学組成分析、化学的風化
Anma et al. (1990), Oshima et al. (1997) and others, who have drilled borehole cores in Lake Suwa and its shore, examined the paleoenvironmental change using pollen and diatom fossil assemblages. These studies discussed the paleovegetation and aquatic environment and tried to access the climate condition. On the other hand, approaches from the clastic materials themselves, such as historical changes in the depositional environment and the source area, have not been studied. In particular, the chemical composition of the sediments is likely to record the average rock assemblage of the source area and the temporal changes in its weathering and alteration. In this study, the deposits recovered from the southern shore of Lake Suwa were examined the chemical weathering condition and source materials assemblage using the chemical analysis.
The ST2020 core was drilled near the mouth of the Miyagawa River, and the SK2021 core was drilled near the mouth of the Kamikawa River at the Lake Suwa in central Nagano Prefecture. The sedimentary facies recorded in both cores shows that the drilled sites changed from river channel and floodplain environments to lake bottom and finally back to river channel environments. The age of the deposits is estimated to be about 25000 years ago at the bottom of ST2020 based on 14C dating. The core lithology was divided into five units. Unit1 (25000-13400 years ago), Unit2 (13400-12100 years ago), Unit3 (12100-10000 years ago), Unit4 (10000-4600 years ago), and Unit5 (4600 years ago to present).
A total of 208 samples from both cores were analyzed for chemical composition by X-ray fluorescence (XRF) analysis, and 25 samples from the ST2020 core were used to identify clay minerals by X-ray diffraction (XRD) analysis.
As a result, most of the samples showed the low intensity of XRD, which is suggestive of domination of amorphous materials, such as volcanic glass and immature soils, which are distributed in the present-day Kamikawa River, flowed from the foot of Mt. Yatsugatake. However, Unit3 showed higher intensity of kaolinite in XRD analysis than one of the other strata. The Al2O3/TiO2 ratio, which is an elemental ratio that consists of not easily lost during chemical weathering, was used as a source indicator. The higher values of the Al2O3/TiO2 ratio in Unit 4 and the upper part of Unit 3 suggest that slightly siliciclastic rocks were distributed in the source area.
As an indicator of chemical weathering, the CIA value (Nesbitt and Young, 1982) was used. Similarly, the SIW value (Amada and Okaya, 1989), which shows the degree of weathering based on the degree of dissolution of Na, was also examined. The CIA and SIW values were higher in Unit 4 and the upper part of Unit 3, suggesting that the site was subjected to strong chemical weathering during this period. Unit 3, which shows a characteristic earthy color, has a higher CIA value, and higher content of kaolinite, suggesting that the source consists of more felsic rocks. Because the age of Unit 3 is corrected to the period around the Younger Dryas, it may record the effects of global climatic changes during this period.
The ST2020 core was drilled near the mouth of the Miyagawa River, and the SK2021 core was drilled near the mouth of the Kamikawa River at the Lake Suwa in central Nagano Prefecture. The sedimentary facies recorded in both cores shows that the drilled sites changed from river channel and floodplain environments to lake bottom and finally back to river channel environments. The age of the deposits is estimated to be about 25000 years ago at the bottom of ST2020 based on 14C dating. The core lithology was divided into five units. Unit1 (25000-13400 years ago), Unit2 (13400-12100 years ago), Unit3 (12100-10000 years ago), Unit4 (10000-4600 years ago), and Unit5 (4600 years ago to present).
A total of 208 samples from both cores were analyzed for chemical composition by X-ray fluorescence (XRF) analysis, and 25 samples from the ST2020 core were used to identify clay minerals by X-ray diffraction (XRD) analysis.
As a result, most of the samples showed the low intensity of XRD, which is suggestive of domination of amorphous materials, such as volcanic glass and immature soils, which are distributed in the present-day Kamikawa River, flowed from the foot of Mt. Yatsugatake. However, Unit3 showed higher intensity of kaolinite in XRD analysis than one of the other strata. The Al2O3/TiO2 ratio, which is an elemental ratio that consists of not easily lost during chemical weathering, was used as a source indicator. The higher values of the Al2O3/TiO2 ratio in Unit 4 and the upper part of Unit 3 suggest that slightly siliciclastic rocks were distributed in the source area.
As an indicator of chemical weathering, the CIA value (Nesbitt and Young, 1982) was used. Similarly, the SIW value (Amada and Okaya, 1989), which shows the degree of weathering based on the degree of dissolution of Na, was also examined. The CIA and SIW values were higher in Unit 4 and the upper part of Unit 3, suggesting that the site was subjected to strong chemical weathering during this period. Unit 3, which shows a characteristic earthy color, has a higher CIA value, and higher content of kaolinite, suggesting that the source consists of more felsic rocks. Because the age of Unit 3 is corrected to the period around the Younger Dryas, it may record the effects of global climatic changes during this period.