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

采用仪器中子活化分析（INAA）方法，测定了日本东部沿着东京湾（II）（凹处）、东京湾（IV）（口部）、陆津湾和丰加湾以及西北太平洋海盆沉积物中14种常量和微量元素的含量。用~（13）Pb法计算了近岸沉积物柱样的沉积速率和沉积年龄<210>。本研究的主要结果如下：1.稀土元素丰度模式表明，沉积物的主要来源是陆源物质。La^*/Lu^* 和Ce^*/La^* 比值表明，东京湾（II）和陆津湾的沉积物比其他地区更能直接反映河流的贡献。此外，Th/Sc比值表明，滨海沉积物主要来源于火山岛弧、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比值之间的相关系数在0.920 ~ 0.991之间，表明东京湾（II）和丰贺湾的沉积物均处于还原状态，而东京湾（IV）和陆津湾的上层沉积物至少处于氧化状态.东京湾（II）的沉降速度约为东京湾（IV）的2倍，因此，东京湾（II）的沉降机制和沉降环境与东京湾（IV）明显不同。计算了丰加湾第5层（8 ~ 10 cm）的沉积年龄，其年代约为1940 ~ 1950年，与乌苏火山喷发形成昭和神山的年代（1943 ~ 1945年）相一致。

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.