长江口外砂泥过渡带沉积粗化趋势与成因分析

    Sediment Coarsening Trend and Genetic Analysis of the Sandy-muddy Transitional Zone off the Yangtze River Estuary

    • 摘要: 长江口外存在一狭长的砂泥过渡带将长江水下三角洲泥质区与东海陆架砂质区隔开,其对长江入海泥沙量和陆架海洋动力变化非常敏感。本文分析了近十年来长江口外表层沉积物粒度变化,以及砂泥过渡带YE16站位连续两年获取的柱状样沉积物粒度、特征元素比值、有机元素和放射性核素组成特征。结果表明,近期砂泥过渡带外边界不断向陆移动,表层沉积物粗化明显,YE16站位近年出现顶部砂质层直接覆盖在底部泥质层上,且砂质层厚度增加较快。基于粒级-标准偏差法与组分分析法的粒度端元(EM)特征发现,YE16柱状样EM3端元(中细砂端元)占比显著升高,以EM3为主的沉积物具有较低的C/N比值和较高的Sr/Ba、Ca/Ti比值,210Pb和137Cs放射性活度低,指示其母源沉积物年龄较老且以海源生源组分占主导。综合分析认为,EM3端元占比增加可归因于强风浪等高能事件驱动下,中陆架变余砂质沉积发生再悬移和向岸搬运所致。这一外源输入机制与长江入海泥沙锐减共同构成了沉积物粗化的双重驱动:前者通过砂质沉积的外源补偿直接增加粗粒沉积占比,后者通过减少泥沙供应削弱了细粒沉积对粗化的稀释效应,两者共同导致了长江口外砂泥过渡带近期向陆迁移、表层沉积物粗化和砂质层增厚等现象。在极端气候事件和流域人类活动影响不断增强背景下,长江口外砂泥过渡带变迁及其环境效应值得持续关注。

       

      Abstract: A narrow sandy-muddy transitional zone exists offshore the Yangtze River estuary, separating the muddy subaqueous delta from the sandy East China Sea shelf, which is highly sensitive to the changes in fluvial sediment supply and marine hydrodynamic dynamics. We investigated decadal-scale changes in surface sediment grain size offshore the estuary and examined two sediment cores in YE16 site collected in two consecutive years from the transitional zone, focusing on grain size, elemental ratios, organic compositions, and radionuclides. Results reveal a recent landward migration of the outer boundary of the transitional zone, obvious surface sediment coarsening, and rapid thickening of sandy layers overlying muddy deposits in YE16 site. Grain-size end-member (EM) analysis via the grain size-standard deviation method and component partitioning identified a pronounced increase in EM3 (medium-fine sand end-member) in YE16 cores. Sediments dominated by EM3 display lower C/N ratios, elevated Sr/Ba and Ca/Ti ratios, and depleted excess 210Pb and 137Cs activities, indicating older provenance with marine biogenic dominance. Integrated analysis attributes the EM3 increase to resuspension and onshore transport of relict mid-shelf sandy sediments driven by high-energy events (e.g., storm waves). This mechanism of exogenic inputcoupled with drastic reductions in Yangtze-derived sediment, forms a dual driver for coarsening: exogenic sand compensates coarse fractions directly, while diminished fine sediment supply weakens the dilution effects. These combined processes explain recent landward migration, surface coarsening, and sandy layer thickening. Against intensifying extreme climatic events and watershed human activities, ongoing monitoring of the transitional zone’s evolution and environmental impacts is imperative.

       

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