琵琶湖底表層堆積物の粒径分布  [in Japanese] Grain Size Distribution of the Surface Sediments in Lake Biwa, Japan  [in Japanese]

2km間隔の164地点で採取した5〜10cmの深さおよび10〜15cmの深さの堆積物について改良した比重計法で粒度分析をおこなった。琵琶湖全域にわたる系統的な粒度分析はこれが最初である。中央粒径値をもとにした粒径分布図には, 内側ほど細粒になり, 最中心部で少し粗粒になるという特異な環状の粒径分布が安曇川河口の沖合いに認められた。その位置は最近実測された第1環流の位置に対応しており, その粒径分布は, 垂直循環を伴なった収束する環流による運搬と選別の作用として説明ができる。北湖の沿岸域では, 水深10m前後まで砂質堆積物が分布している, 姉川河口域の沖合いでは, 北西と南東に伸びた砂質泥の分布が認められ, 温度成層期と非成層期の河川流入水の流れに対応したものと考えられる。堆積物の粒径分布は湖水の平均的な運動を反映したものと考えられる。

Grain size analysis by the modified hydrometer method was performed on the bottom surface sediments sampled from 164 localities every 2 km interval in Lake Biwa. The systematic survey enable to make up a grain size distribution map of the lake for the first time. Sandy sediments coarser than 4 phi in median size is limited to the zones shallower than about 10 meters, and the most area of the lake is covered by clayey silt or silty clay which is finer than 6 phi in median diameter. Based on the median size, there appears a concentric pattern of grain size distribution between the mouths of Ado and Amano Rivers, about the central portion of northern basin of North Lake Biwa. The grain size become finer in the inner zone of the concentric circle, but the center is a slightly coarser than the surrounding parts. The location of the concentric circle well corresponds to that of the circular current, Gyre I, which was revealed recently by the water temperature distributions and the radar trackings of drifters. Convergent circular currents of Gyre I in the epilimnion, can explain well the concentric distribution of sediments. The grain size and modal analyses of the surface sediments at 39 localities near the mouth of Ane River revealed that there may be two directions of the inflows from the river to the lake, namely, northwest and southeast. The former seems to be dominant in thermal-stratified seasons, and the later in non-stratified seasons. The grain size distribution of lake sediments should play an important role to clarify the major movements of lake water.