抄録
A density gradient centrifugation method by using Thoulet's liquid (HgI2/KI : density 1.92.4g/cm3) was applied for separating autochthonous and allochthonous materials in lacustrine sediments.
The measurement of density distribution of sediment samples showed that a sediment sample can be divided into three main fractions in terms of density : Fraction 1 (density : <1.9g/cm3), which consists of wood, leaf fragments and sometimes phytoplanktons (nearly living?); Fraction 2 (density : 1.92.2g/cm3), which is predominantly composed of diatoms whose organic constituents seem to have been considerably lost; Fraction 3 (density : >2.2g/cm3), which consists of microscopically shapeless materials with a much lesser amount of diatoms. The former materials are probably flocks of clay minerals and chemical deposits.
It was shown that the vertical change of carbon/nitrogen ratio in sediment of Lake Yunoko is closely related with that of relative amounts of the three fractions with different density. The weight percentage of autochthonous materials in the total was estimated to be 80 and 22 for the near surface sediment sample of Lakes Haruna and Aoki-ko, respectively.
