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  • Sr isotope ratios of sediments in southern Lake Biwa

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A sediment core in the southern Lake Biwa was analyzed for ^<210>Pb, ^<137>Cs and Sr isotope ratio. The sedimentation rate of the lake sediments assumed by ^<210>Pb is about 3mm/y. 137Cs, a fallout product from atmospheric nuclear weapon tests, shows maximum activity at 9.5cm depth. Maximum fallout of ^<137>Cs to water and land occurred during 1963-1964,and the sedimentation rate assumed by ^<137>Cs is also about 3mm/y. The sedimentation rate of 3mm/y, however, could be untrue because surface sediments are disturbed and mixed by physical turbulence. Sr isotope ratios of the lake sediments radically change from 0.7185 to 0.7198,and Sr concentrations also radically change from 73ppm to 64ppm at near 10cm depth. Therefore, the following causes are expected : 1) Sr system has changed by artificially environment pollution since near 1960,and 2) surface sediments are disturbed and mixed by physical turbulence. In the case of 2), the profiles of ^<210>Pb and ^<137>Cs lead to mixing process except sedimentation process. The profiles of other nuclides such as ^<14>C and ^<90>Sr would help better understanding of the behavior of lake sediments. It is necessary to analyze another sediment cores of southern and northern Lake Biwa.



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