Lake Erhai is the seventh biggest lake of China and locates in Dali City, Yunnan Province in the southwestern part of China. A sediment core was taken at the center of the lake (25°48′N, 100°11′E; water depth 21m) by the thin-wall type piston core sampler. The core length is 42.63m and the recovery is almost 100%. This study clarified the formational environment of vivianite concentration in the lake sediments on the basis of sedimentological features and geochemistry analyses. Based on detailed description of the core, the sediments consist of homogeneous gray to dark gray silty clay with bedded and lens-type vivianite (Fe₃(PO₄)₂·8H₂O) concentration. These concentrations develop at the depths of 42.5 to 40, 37.5 to 36, 34.5 to 31.5 and 24 to 20m. From the evidence of sedimentary facies, SEM images and mineral components, these concentrations may have formed under an anoxic condition at the bottom of water column. The results of geochemical analyses show decrement of quartz content and of Al/Ti ratio, which indicates decrease of detritus supply, as well as increment of total inorganic carbon content, in every such interval. This correlation suggests that vivianite precipitated at the bottom of the lake under a warm and humid climate which could have caused an anoxic bottom condition due to the occurrence of the thermocline. Hence, bedded and lens-type vivianite concentrations in lacustrine sediments can be a good proxy for ancient lake water condition and paleoclimates.

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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.

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