Paleoenvironmental change reconstructed from varved lacustrine sediment in Lake Fukami, Nagano Prefecture, central Japan

Continuous varve sequences were taken in 2000 from Lake Fukami, a monomictic lake in the southern part of Nagano Prefecture, central Japan. A total of 196 varve units were recognized in the uppermost 2m interval of those sequences. The stratigraphic changes in the sedimentation rate have some high peaks, which indicate the increment of clastic particles supplied to the lake owing to natural disasters like floods or earthquakes as well as artificial constructions around Lake Fukami. As a result of correlation between the meteorological records at the nearby Iida meteorological station and the variations in total carbon flux in Lake Fukami based on the varve chronology, it is clarified that the total carbon flux is in inverse proportion to the February mean temperature and precipitation. These indicate that the February mean temperature controls the vertical mixing within the lake water column that affects the productivity of either phytoplankton in the epilimnion or decomposition of organic matter in the hypolimnion. Furthermore, the February precipitation probably affects the activity of denitrifying bacteria during the stagnation periods. Mineralogical investigation detected pyrite and siderite minerals in the sediment by X-ray powder diffraction analysis and X-ray fluorescence analysis. Although either of these iron minerals can precipitate in disaerobic water condition, only pyrite precipitates under sulfate ion rich condition. Considering the physicochemical features in Lake Fukami, pyrite was deposited during the stagnation period from spring to autumn, whereas siderite precipitated during the circulation period in winter.