Study on sedimentary environment and mechanism investigated through marine sediment core samples

采用仪器中子活化分析（INAA）对东京湾（II）（凹槽）、东京湾（IV）（河口）、木津湾和丰卡湾以及西北太平洋海盆等日本东部沿海的14种海相沉积物岩心样品进行了测定。采用^<210>Pb法计算了海岸沉积物岩心的沉积速率和沉积年龄。本文的研究结果总结如下：1。稀土元素丰度模式表明沉积物的主要来源为陆源物质。La^*/Lu^*和Ce^*/La^*比值表明东东湾（II）和木津湾沉积物比其他地区更直接地反映了河流的贡献。此外，Th/Sc比值表明，沿海沉积物主要来源于火山岛弧和日本列岛，而西北太平洋沉积物主要来源于大陆。Ce/U和Th/U的相关系数在0.920 ~ 0.991之间，表明东京湾（II）和丰卡湾的沉积物全部处于还原状态，而东京湾（IV）和木苏湾的上层沉积物至少处于氧化状态。东京湾（II）的沉积机制和沉积环境明显不同于东京湾（IV），因为东京湾（II）的沉积速率大约是东京湾（IV）的两倍。丰卡湾第5层（深度8 ~ 10 cm）的沉积年代约为1940 ~ 1950年，与乌苏火山喷发形成昭和新山的时间（1943 ~ 1945年）一致。

Fourteen major and trace elements in marine sediment core samples collected from the coasts along eastern Japan, i.e. Tokyo Bay (II) (the recess), Tokyo Bay (IV) (the mouth), Mutsu Bay and Funka Bay and the Northwest Pacific basin as a comparative subject were determined by the instrumental neutron activation analysis (INAA). The sedimentation rates and sedimentary ages were calculated for the coastal sediment cores by the ^<210>Pb method. The results obtained in this study are summarized as follows:1. Lanthanoid abundance patterns suggested that the major origin of the sediments was terrigenous material. La^*/Lu^* and Ce^*/La^* ratios revealed that the sediments from Tokyo Bay (II) and Mutsu Bay more directly reflected the contribution from river than those of other regions. In addition, the Th/Sc ratio indicated that the coastal sediments mainly originated in the materials from the volcanic island-arcs, Japanese islands, whereas those from the Northwest Pacific mainly from the continent.2. The correlation between the Ce/U and Th/U ratios with high correlation coefficients of 0.920 to 0.991 indicated that all the sediments from Tokyo Bay (II) and Funka Bay were in reducing conditions while at least the upper sediments from Tokyo Bay (IV) and Mutsu Bay were in oxidizing conditions.3. It became quite obvious that the sedimentation mechanism and the sedimentation environment at Tokyo Bay (II) was different from those at Tokyo Bay (IV), since the sedimentation rate at Tokyo Bay (II) was approximately twice as large as that at Tokyo Bay (IV). The sedimentary age of the 5th Iayer (8〜10 cm in depth) from Funka Bay was calculated at approximately 1940-50, which agreed with the time, 1943〜45 when Showa-shinzan was formed by the eruption of the Usu volcano.