长江口水下三角洲北部近百年沉积物粒度组成及其对水动力环境的响应

Grain Size of Recent Sediments in the North of Changjiang Subaqueous Delta and Its Implication of Estuarine Hydrodynamics

随着长江入海泥沙减少,长江三角洲响应问题越来越受到关注。本文选取对水动力变化较为敏感的水下三角洲与陆架过渡区域,通过对钻孔沉积物进行210Pb和137Cs同位素定年和粒度参数分析,并提取敏感粒级含量及平均粒径垂向变化,尝试探讨其对近百年来长江河口河势变化以及入海泥沙减少的响应。研究发现1954年之前,该区沉积物颗粒较细,粒度参数特征接近现代河口泥质区沉积物,敏感粒径特征反映出河流作用影响较强,说明当时处于泥质区沉积范围;之后沉积物明显粗化,参数特征向陆架残留砂过渡,海洋动力的影响明显增强,并带入陆架粗颗粒物质,说明当时泥质区南移,该区处于河口和陆架沉积过渡区。推测这一变化主要和1954年后长江口北支河道萎缩有关,该区从长江水沙覆盖范围内变为水沙向海输运的边界上,海洋动力对沉积物改造加剧,导致沉积物粗化。同时,并未发现该区沉积物对1980年代后长江入海泥沙显著下降有所响应。近期发现的长江口外泥—砂分界线的西移很可能也包含着河口河势变化而导致的沉积物粗化的贡献。

The response of Changjiang delta to river sediment declination has attracted more and more attention in recent years. The present paper analyses radiometric210 Pb and137Cs and grain size of the core on the north of the subaqueous delta and extracts sensitive grain size population,which help explore the response of the delta to channel regime evolution and river sediment declination. The radioactive nuclear results show mean sedimentation rate of 3.28 cm / yr for the whole core by210 Pb. This rate is much higher than 1.78 ~ 1.90 cm /yr by137 Cs for the upper core section( <104 cm),mainly because of different deposit environments in the two sections. Combined with the results of210 Pb,137Cs and spherical carbon particle in the previous study,sedimentation rate of the upper core section is about 1.85 cm /yr. Grain size results show that from finer sediments with the same feature as surface sediments of mud region in the Changjiang estuary control the lower core section before 1954,which implies the northern boundary of mud region beyond this core at that time. The sediments become coarsening upward the core after 1954,which feature are close to shelf sediments. The above environmental change might be closely related to the block of north branch in the estuary after 1954,when it was away from influence of Changjiang freshwater and sediments became coarser due to reformation and join of continental material by marine dynamics. However,in terms of sediment composition,we did not find an obvious response to significant sediment supply decline of the Changjiang after 1980's. Therefore,besides sediment supply declination,coarsening due to estuarine channel regime evolution might be a contributor to western retreat of clay-sand boundary off the Changjiang estuary in the recent studies.