Chronology and sedimentation process of varved lacustrine sediment in Lake Fukami, central Japan

摘要

Continuous varve sequences were taken from Lake Fukami, a monomictic lake, southern part of Nagano Prefecture, central Japan in 2000. Total 196 varve units were recognized in the upper 2 m horizon of the sediment core. Sedimentation rates in the core have some high peaks, indicating increments of clastic particles supplied to the lake owing to natural disasters including floods and earthquakes, and the artificial construction around Lake Fukami. The results of the correlation between the meteorological records in lida meteorological station and the total carbon flux change based on the varve chronology, indicates that the total carbon flux shows an inverse relationship to February mean temperature and precipitation. February mean temperature controlled the vertical mixing of lake water column that affects the productivity of phytoplankton in the epilimnion or decomposition of organic matter in the hypolimnion. February precipitation probably has affected the activity of denitrifying bacteria in the stagnation period. Pyrite and siderite minerals are recognized in the core by observation of thin sections and X-ray powder diffraction analysis. The pyrite was deposited in summer, and the siderite was deposited in summer and winter. These iron minerals indicate the decomposition processes of organic matters in the water column in each season.