Sedimentary transition of the Yangtze subaqueous delta during the past century: Inspiration for delta response to future decline of sediment supply
Sedimentary transition of the Yangtze subaqueous delta during the past century: Inspiration for delta response to future decline of sediment supply
复制标题
长江水下三角洲百年沉积转型：三角洲应对未来沉积物供给减少的启示
DOI：
10.1016/j.margeo.2020.106279
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发表时间：
2020
期刊：
影响因子：
2.9
通讯作者：
Chen Zhongyuan
中科院分区：
文献类型：
--
作者：
Zhan Qing;Li Maotian;Liu Xiaoqiang;Chen Jing;Chen Zhongyuan
长江三角洲如何应对沿江泥沙减少已成为当前研究的热点和亟待解决的问题，尤其是在2003年三峡大坝蓄水后。通过对整个长江水下三角洲表层样品和短岩心的沉积物粒度进行研究，旨在更好地了解水下三角洲对泥沙供应减少和河口局部水动力调整的空间响应。结果表明，从河口到陆架分布有河口砂坝(I区)、前三角洲(II区)、三角洲-陆架过渡带(III区)和残留沙(IV区)4个沉积带，其特征是从I区到II区先细化，然后从II区粗化到IV区。在过去的几百年里，水下三角洲在空间上普遍遵循这种沉积体系向海推进，表现为主要水下三角洲的核心普遍向上细化。然而，沉积物粗化发生在三角洲-陆架过渡带(III区)的岩芯顶部，这意味着最近的退变过程。根据~(210)Pb和~(137)Cs的分析结果，沉积由进积向退积的转变发生在20世纪50年代以后，80年代以后，三区北部的沉积加剧，主要是由于北支的河道萎缩和河流泥沙的下降所致。值得注意的是，三区南部的表层粗化可能与2003年三峡大坝合闸以来泥沙的急剧减少有关，这意味着长江水下三角洲可能已处于全面的沉积过渡阶段。北部水下三角洲的沉积过渡可以为未来长江三角洲对泥沙供应进一步减少的沉积响应提供有洞察力的参考。
How the Yangtze delta responds to riverine sediment decline has become a hot research topic and an urgent issue to be addressed, especially after the impoundment of the Three Gorges Dam in 2003. This study investigated sedimentary features through the sediment granularity of surficial samples and short cores over the entire Yangtze subaqueous delta, aiming to better understand the spatial response of the subaqueous delta to sediment supply decline and local hydrodynamic adjustment of the estuary. The results show that four sedimentary zones of estuarine sand bar (Zone I), prodelta (Zone II), delta-shelf transition zone (Zone III) and residual sand (Zone IV) are distributed from the estuary to the shelf, characterised by first fining from zone I to zone II, and then coarsening from zone II to zone IV on grain size of surficial sediments. In the past few hundred years, the subaqueous delta has been generally prograded seaward following this sedimentary system spatially, manifested by generally fining upwards in the cores of the main subaqueous delta. However, sediment coarsening occurs on the top of the cores in the delta-shelf transition zone (Zone III), implying a recent retrogradation process. Based on the results of210Pb and137Cs, the sedimentary transition from progradation to retrogradation occurred since 1950s and was intensified after 1980s in the north part of Zone III, mainly caused by channel shrinking of the North Branch and riverine sediment decline. Comparatively, such a sedimentary transition is not significant in the south part of Zone III. But it is worth noting that surficial coarsening in the core (of the south part of Zone III) is probably related to drastic sediment decline since 2003 when the Three-Gorges Dam closed, which means that the Yangtze subaqueous delta has probably been in a full sedimentary transition. The sedimentary transition of the north subaqueous delta can provide an insightful reference for the sedimentary response of the Yangtze delta to further decline of sediment supply in the future.