Varved lacustrine sediments and diatom assemblages of Lake Fukami, central Japan

A sediment core, taken from Lake Fukami in southern Nagano Prefecture in the summer of 1999, contained alternating light- and dark-colored laminae. Diatom microstratigraphy was determined for the succession between 60 and 68 cm core depth. The frequency of Synedra spp. varied from more than 40% in the light-colored laminae to less than 10% in the dark laminae. Aulacoseira spp. or Cyclotella (?) spp. were dominant in the dark laminae, in which the proportion of Synedra spp. was relatively low. The correlation of these diatom assemblages with alternating lamina color indicates a seasonal succession within the diatom biota. Each couplet of light- and dark-colored laminae in the sediment is, therefore, considered to be an annual sedimentation increment.