Depositional characteristics of sediments in the Changjiang River subaqueous delta during the catastrophic flood in 2020
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摘要:
大河特大洪水会对河口地区沉积环境产生显著影响，然而当前有关河口三角洲古洪水沉积特征的认识存在较大争议，有待通过现代洪水沉积研究深入揭示河口三角州洪水沉积特征。本文于2020年长江流域性特大洪水发生期间，在长江水下三角洲采集了16根短柱样，在实验室进行了粒度和有机地球化学指标（TOC、TN）分析。结果表明短柱中洪水期间产生的沉积层厚度为3～21 cm，上部洪水沉积层TOC含量平均值为0.59%，TN含量平均值为0.077%，与下部常态沉积层相比均有所增加。洪水层沉积物的平均粒径（13.23 µm）比其下部常态沉积物平均粒径（13.87 µm）略微偏细。代表洪水事件沉积的粒度端元组分EM1和研究区以往钻孔中洪水沉积粒度结果的对比表明，2020年长江口洪水沉积相对底部常态沉积粒度偏细，但比以往研究区钻孔中的洪水沉积粒径偏粗，这与传统古洪水沉积以粗颗粒组分为主的认识存在不同，这应该是流域内大坝建设对沉积物的圈闭作用、中下游河道侵蚀作用和水下三角洲受海洋动力侵蚀作用共同导致。该研究对于河口地区长时间尺度古洪水事件序列的重建以及极端水文事件沉积记录解译具有重要的科学意义。
Abstract:Catastrophic floods in large rivers exert significant influences on the sedimentary environment in the estuarine region. However, viewpoints of sedimentary characteristics of the paleoflood in the estuarine delta is controversial. Therefore, it is necessary to understand the sedimentary characteristics of modern flood. The whole Changjiang River basin suffered from a catastrophic flood in 2020. Sixteen sediment cores in the Changjiang subaqueous delta were collected during the flood period. Grain size and organic index (TOC, TN) were measured. The result indicates that the flood layers is 3～21 cm thick, the average TOC and TN is 0.59% and 0.077%, respectively, which is higher than that of lower part deposit. The mean grain size of the flood layers (13.23 µm) is finer than that of the lower part deposits (13.87 µm). The end-member modelling analysis indicated that the finest populations (EM1) were originated from the 2020 flooding. Comparing the EMs of 2020 flood deposits with grain size characteristics of flood deposits in previous studies, we found that the 2020 flood sediments were finer than the lower part non-flood deposits, but coarser than the previous flood sediments, which is different from the common view that the paleoflood deposits are characterized by coarser sediments. We believed that this difference was due to (1) human activities (e.g. dam construction) in the river basin, by which more coarse sediment were trapped within the dam; (2) the weakened erosion in middle and lower reaches of river channels; and (3) the subaqueous delta by ocean dynamic erosion. This study is beneficial for the reconstruction of long-term paleoflood events sequences and the interpretation of extreme event deposition in the estuarine region.
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