We have tried to clarify the influence of diatom tests on the grain size distribution of muddy lacustrine sediments measured by laser diffraction size analyzer on the basis of comparison between silica-dissolved sample and silica-undissolved sample or the same sediment. Tested sediments are two cores taken from the center or Lake Kizaki, Nagano Prefecture. Bimodal distribution or grain size is common feature or the sediments. Coarser mode centered around 150 -200 microns become more distinct and is increased in ratio after diatom dissolution. This would be explained by the fact that diatom tests of small size are abundant and affect to increase the ratio of finer mode of grain size distribution. In addition, when the tests of diatom tests such as Cyclotella, mostly around 10-20 microns in size, are predominant, the peak or the finer mode moves to coarser side after biogenic silica dissolution. On the contrary, when large elongated diatom such as Fragilaria and Synedra, mostly 60-80 micrometers in length, are abundant, the peak of finer mode moves to finer side after silica dissolution. On the basis or grain size analysis or diatom-dissolved sediments, finer mode or the sediments from Lake Kizaki seems to be transported by suspension, and coarser mode may deposited from underflow such as density current, because sandy particle can not be transported to the lake center in suspended condition. Sedimentation rate at the lake center is estimated as 0.8cm/year. Small underflow might take place more than once per year.We can recognize several coarser horizons in the Gored sediments which have distinct coarser mode and clear peak or grain size. The interval or coarser horizons corresponds with decades or years, suggesting an average interval of large flood.

レーザー回折散乱型粒度分析装置を用いた粒度分析に関して,生物源シリカ粒子を溶解させたものと溶解させないものとを比較した.長野県の木崎湖湖心部で採取した2本のコアについて研究した.粒度分布は双峰性を示す特徴があり,生物源シリカ粒子溶解後には粗粒側のモードが明瞭になり,その比率も増加する.それは細粒側のモードに多くの生物源シリカ粒子が存在するためである.またCyclotellaが優占する層準では,生物源シリカ粒子の溶解後に,細粒側のモードはより粗粒側に移動し,反対にFragilariaやSynedraが優占する層準ではより細粒側に移動する.木崎湖の粒度分布の細粒側のモ-ドは浮遊状態で,粗粒側のモードは底層流によって堆積したと考えられる.砂質粒子は密度流以外では湖心部まで至らないためである.小規模な底層流は1年に1回程度の頻度で発生している.粗粒側のモードが卓越する層準は10-20年ごとにあり,大きな洪水間隔を示唆する.

Article

堆積学研究. 65:15-27 (2007)