Symposium II: Acquirable qualifications at Departments of Geography

金沢大学人間社会研究域人間科学

Paleointensity of the asian winter monsoon and the westerlies since the last glacial period, reconstructed by eolian dust flux in lacustrine sediments of lake biwa and lake suigetsu, central japan

Lacustrine sediments of Japan contain a lot of eolian dusts transported from Chinese Loess Plateau by the Asian winter monsoon and the westerlies. Sequential changes of eolian dust concentrations in lacustrine sediments since the last glacial period would be useful as a proxy record for reconstructing paleointensity of the Asian winter monsoon and the westerlies. In this paper, it is clarified that the changes of eolian dust flux during the last 38,000 years in the lacustrine sediments of Lake Biwa and Lake Suigetsu, central part of Japan. Sedimentological observations and calculation of sedimentary flux in lacustrine sediment cores of both lakes show that most of illite and quartz minerals in lacustrine sediments originate in eolian dusts from the inland of China, and that the intensity of the Asian winter monsoon and the westerlies has strengthened when eolian quartz flux was higher and has weakened when flux was lower. These changes of the intensity of the Asian winter monsoon and the westerlies have been closely related to the global climatic changes. In the Lake Biwa sediments during the last 38,000 years, it can be recognized such global climate coolings as Heinrich events 1-4, Younger Dryas and 8.2 ka cooling event. In the Lake Suigetsu sediments. Younger Dryas and Older Dryas cooling periods can be recognized. Also, our investigations suggest that the changes of the intensity of the Asian monsoon and the westerlies might be one of the triggers of the global environmental changes

Changes of eco-systems in the last 500 years caused by human impacts in lake suigetsu, gentral japan

Annually laminated (varved) lacustrine sediments have been found in many Japanese lakes including Lake Suigetsu. Several studies for the reconstruction of past environmental and climatic changes have been carried out with varved sediments. However we have not yet clarified the relationships among abrupt changes of environment, biologic changes in lake eco-systems, and organic and inorganic changes in bottom sediments. Two problems can be summarized as follows: 1) on authigenesis of minerals as main components of sediments and 2) on ecological response of living microplanktons to changes of lake water condition. By reason of confirming how the chemical composition of authigenic minerals and the species composition of diatom assemblages in varved sediments are linked with chemical composition of lake water, we carried out new coring and took 8 cores of well-preserved varved sediments by Meckereth piston sampler at Lake Suigetsu in 2000. Based on varve chronological, sedimentologic and micropaleontologic investigations, we clarified the following facts: 1) most of authigenic mineral particles have precipitated directly from bottom water mass through sulfate reduction, but not from interstitial water in sediments: 2) abrupt environmental changes in water eco-system were caused by human impacts in and around Lake Suigetsu during the last 500 years: 3) after the lake eco-system gradationally evolved by low rate external impacts, it never returned to the initial condition without other impacts: 4) short term changes of lake eco-system and phytoplankton communities have been caused by external human impacts with high rate

Varved lacustrine sediments of lake tougou-ike, western japan with reference to holocene sea-level changes in japan

Holocene laminated sequence consisting of alternations of light- and dark-colored laminae was recognized in the sediment core taken from Lake Tougou-ike in 1992 by our research group. Biochemical and mineral compositions. micro-sedimentary structures and AMS ^＜14＞C dating of organic remains in this laminated sequence support the laminated sediments are identified as non-glacial varves, which were formed mainly by seasonal changes of diatom productivities. Varve counting and ^＜14＞C dating provided Holocene detailed chronology, from annual to decadal for reconstructing paleoenvironments from the sediment core with high time resolution. Based on this chronology Holocene relative sea-level changes at Lake Tougou-ike were estimated from authogenic iron mineral contents of the sediments. The relative sea-level changes show abrupt rises at about 8,900, 7,700, 5,600, 4,000, 3,600 and 2,700 calendar years BP. with gradual falling between the interval periods. These relative sea-level changes are suggeste